Exhaust muffler



F. H. SCHNELL EXHAUST MUFFLER Feb. 9, 1932.

Original Filed May 8, 1929 HNVENTOR Eb. sc/me ATTORNEYS BY FM %W,

Patented Feb. 9, 1932 UNITED STATES PATENT OFFICE FREDERICK SGHNELL, OF MADISON, WISCONSIN, ASSIGNOR T0 0. F. BURGESS LABORA- TORIES, INC., 013 MADISON, WISCONSIN, A CORPORATION OF DELAWARE EXHAUST MUFFLER Original application filed Kay 8, 1929, Serial No. 361,376. Divided and this application filed May 29, 1929. Serial No. 366,891.

My invention relates to mufllers for noise-- producing gases and more particularly to exhaust mufilers for'expanding and noiseproducing pulsating gases such as are discharged from internal combustion engines.

This application is a division of my copending application Serial No. 361,37 6, filed May 8, 1929 (now Patent No. 1,811,762, granted June 23, 1931).

My improv'ed exhaust mufller departs very decidedly in construction from the exhaust mufilers heretofore employed.

The explosion within the cylinder of an internal combustion engine creates tremendous pressure therein. The steam or vapor in the cylinder of a steam engine is also under very high pressure. Upon the opening of the exhaust port, the gas emerges at high velocity, very quickly reducing the pressure in the cylinder. By the word gas, I mean to embrace both gas and vapor. The result is a succession of pulses of gas at high pressure traveling through the exhaust manifold or pipe or any other type of exhaust system at a high velocity. Upon'the emergence of the gas pulse into the atmosphere an additional spurt in velocity is probably acquired by reason of expansion. and a sharp sound results. The greater the pressure gradient between the pulse of gas and the atmosphere the greater the velocity and the sharper the sound. In order to successfully silence exhaust noises the muffler must suppress the pressure peaks and thus even the flow of the escaping gas pulses to the point where a sound is not created.- In addition, I have found that sound communicated to or generated within the muffler should be silenced.

The mufilers of the present day accomplish the desired result by any or all of the following methods: cooling the exhaust gases and diminishing their volume; allowing the gases to expand and reducing their pressure while confined; creating eddy currents and internal friction within the gases; causing friction between the gases and the walls and passages within the muffler; and/or impeding the forward progress of the gases by the interposition of bafiles in their path. The typical muffler of today is a metal shell connected in the exhaust system of. an engine which may incorporate one or more of the following features; an expansion and mixing chamber followed by a small outlet; a tortuous passageway; a series of obstacles or baflles in the passageway; and/or a jacket in which a cooling medium is circulated.

Only a limited sphere of application can at best be found for a mufiler with a cooling jacket. The expansion chamber alone is not successful since it approaches the condition of the atmosphere and sound is created therein. The types embodying tortuous paths, obstacles, and battles, retard the flow of gas, and in so doing, build up a back pressure within the mufller and reduce the efiiciency of the engine. There is a muffler being produced which actually assists in the scavenging of the cylinder but the scope of its application is limited to special types of engines of known, constant speed.

It is an object of my invention to provide an exhaust muffler whose scope of application extends to practically all of the common types of explosion and steam engines, to fire arms, and to air intakes and exhausts as in air compressors.

It is a further object of my invention to provide a mufiler which will not build up suflicient back pressure to appreciably reduce the efliciency of the engine.

It is a further object of my invention to provide a mufller which will silence explosion noises more effectively than do the mufflers at present employed.

It is a still further object of my invention to provide a mulfler which is more simple in construction than are the present mufilers of commerce.

Other and further objects of my invention will become apparent as the following description progresses, which is to be taken in conjunction with the accompanying drawings, wherein:

Fig. 1 is a longitudinal, sectional view of one form of my invention; and

Figs. 2 and 3 are charts which illustrate the eifect of my mufitling device upon exhaust gases.

Fig.1 represents a form of my invention embodying the open expansion chamber idea. Outer cylindricalshell 23 is connected, as by welding, to end closure discs 24 and 25 which may be threaded to receive exit and entrance pipes 26 and 27 lVithin shell 23 and coaxial therewith is cone-shaped shell 28 of. perforated metal or other foraminous facing with its large end adjacent the gas entrance end of'the mufiier. The annular space 29 between the two shells is filled with gas-pressure absorbing and sound absorbing material. As the gas-pressure absorbing and sound absorbing material fills the space between the shell 23 and coneshaped shell 28,- it increases in thickness from the inlet end sure absorbing and sound-absorbing material which maybe a mineral fibre, steel, copper or other metallic wool or mixtures thereof,

or any other porous sound-absorbing, nonflammable material such as sized crushed mineral matter, mica, exfoliated vermiculite such as zonolite, blown slag, coke, pumice or other'porous aggregates and built-up units of such aggregates. For silencing'blow-ofi' gases, air compressor intake and exhaust noises, and noises produced by gases at room temperature, flammable gas-pressure absorb-.-

ing and sound-absorbing materials such as wool, cotton, or other cellulosic fibers may be used. a In viewof the fact that every material which is not a perfect reflector of sound is,

to adegree, ,an absorber of sound, itwould probablybewell to draw some sort of a spe ci. fication-for the term sound-absorbing'mat'erial as that term is used the present case. In all acoustic data the soundtransmitted by an open window is used as the'standa'rd of comparison. The ratio of the soundab:

sorbed by anarea of a materialto that trans-- mitted by an equal area of open window is called the absorption factor or value of that material. If a material one squarevfoot in area absorbs one-fourth thesound transmit ted by one square foot of open'window. that material is said to have an absorption factor or valuefof 25 percent. In this specification whenever the term sound-absorption factor or value is used .inconjunction with an absorbing material or construction in a muffler, it is tobe understood that this factor orvalue is; obtained by measuring the absorption of such material or construction 'in the usual way by. means of flat pads and the like of identical material and construetion. This is necessary since it is very difficult or i mpossitble to obtainthe factor after the muffler is assembled. In the mufiiersof able thickness for best results.

my invention as the sound absorption factor of the sound absorbing material decreases, the size of the muffler mustincrease and a material with a factor of 10- percent or less would probably necessitate an unwieldly size. Hence, in the present consideration, by sound-absorbing material I do not contemplate a material having an absorption fac-- tor of less than 10 percent at 1024 double vibrations per second. Throughout this specification all absorption values are based on 1024 double vibrations per second. For most situations an excellent mufller may be constructed with a material having an absorption factor of 25 percent. or more. For exceptionalresults I prefer to use a material having a factor greater than 45 percent. The absorbing material should be of appreci- It' should be at least one-quarter of an inch thick and preferably one-halfinch or more. By mineral fibreI contemplate natural or artificial mineral wool, shredded asbestos, or any other mineral material of the same general nature.

As hereinbefore explained, a series of gas pressure peaks emerges at high velocity from pipe 27. Each peak tends to expand in all manufacturing costs I- prefer circular openings. Holes suitable may vary from approximately 0.070 to 0.125 inches indiam'eter but I do not wish to be limited to these dimensions. For most purposes the holes are evenly distributed throughout the area of thesheet metal although such even distribution is not necessary. The holes are present in sufficient number .so that their total area represents from 2%. percent up to 35 percent of the area of the sheet metal though fair results may be obtained with holes having an area as low as lpercent of the area of the sheet. The -15 shape,' siz e and distribution of the perforations all 'have an influence on the ease with which gases and sound pass through the sheet of metal. The holes should be small enough so that the absorbing material will not work its way out through them when the mufiier is in service. Excellent results are obtained when the ratio of the 'unperforated portion of the metal forming such facing to the openings therein is such that a substantially coni tinuous surface is exposed to the sound waves and gas pressure waves. In such-cases the average dimensions of the individual openings are usually'less than the distance be tween theedges thereof. Tests show that a muflier containing absorbing material faced with a stiff perforated sheet, the perforated area of which is as low as 2 percent of the area of the sheet with holes about .075 inch in diameter, absorbs as much sound as and has a mufliing efficiency equal to a similar muffler in which the perforated facing is omitted. It is possible to omit the perforated metal facing or other foraminous facing by constructing a molded annular or other suitably shaped absorbing material as by bonding sized crushed mineral particles together at their points of contact to form a porous mass. Such a molded gas-pressure absorbing and sound absorbing material has an opening therethrough corresponding to the foraminous tube 28. In another construction metallic fibers, like-steel wool, are formed into a mattress by stitching the fibres together with a metallic thread such as steel wire. This mattress is used to line the inside of the muffler shell 23. The foraminous duct may be omitted when such a mattress is used.

The fibrous gas-pressure absorbing and sound absorbing material comprises nonflammable fibres distributed in heterogeneous arrangement to form tiny interstices, pores or cells of more or less uniform size and dis-- tribution. It is packed in such a manner that usually only from 1 to 20 percent of annular space 2 is occupied by the actual fibres of the material and therefore to 99 percent is free space. The packed fibers, depending on their physical properties, weigh after packing in the muffler, from 4 to'lOO pounds per cubic foot. The packed mineral wool weighs from 9 to 36 pounds per cubic foot, the fibers occupying from 5 to 20 percent ofthe space. Steel wool weighs from 4 to 100 pounds per cubic foot, the fibres occupying from 1 to 20 percent of the space. Especial y good results are obtained with steel wool packed to occupy 2 to 5 percent of the space. If the fibres are packed too loosely they jar down into a more compact mass after the muffler is put into use, and if they. are packed too tightly the gaspressure absorbing and acoustic absorption are cut down, thereby decreasing the muflier efficiency. The absorbing material is subjected to violent pounding and vibration by the oscillating influence of the exhaust gases and must not disintegrate readily under those conditions. Certain types of mineral woolresist this disintegrating action much better than others. However, a metallic wool like steel wool is highly resistant to this action. The gas-pressure absorbing and sound-absorbing portion of the muffler, therefore, pro vides an expansion space for gas-pressure peaks and the maximum pressure of the pulse of gas is decreased as a result of the expansion, the pressure wave being longer and lower in intensity. Fig. 2 shows in solid lines a hypothetical diagrammatic representation of several gas pulses as they enter the mufiier and in dotted lines is a similar representation of the same pulses after expansion has taken place. The pressure wave characteristic has become flattened. I also believe that a second phenomenon takes place within core 29 to further flatten the wave characteristic. Relatively speaking, the rapidly moving gas enters the core in the form of a succession of rapid compressions and rarefactions. The compressions are suppressed and merged together by the choking effect of the walls of the tiny pores or cells with a resulting reduction of the intervening rarefactions. Fig. 3 shows in solid lines the pressure wave characteristic corresponding to the dotted curve of Fig. 2. The dotted lines of Fig. 3 show the pressure wave characteristic after the gas has undergone the choking action of the porous gas-pressure absorbing and sound-absorbing material. The porous material offers practically no resistance to the fairly steady flow of the gas stream through the straight and unobstructed duct 28, and especially if duct 28 is made of smooth perforated, sheet metal, it facilitates the flow with the net result that there is practically no resistance to the gas flow and little or no back pressure is built up aside from that due to the surface friction. The absorption of the gas-pressure peaks by the absorbing material to smooth out the gas fiow and eliminate the noise made by the gas slugs when emerging into the atmosphere is an important feature of my invention. I use the gas-pressure absorbing soundabsorbing material for this double function without appreciably increasing the back pressure. I believe this to be entirely. new in the muflier art.

I believe that some noise is generated at the end of pipe '27 as explained heretofore in connection with expansion chambers. This noise is silenced by the gas-pressure absorbing sound-absorbing material. It is not .projected longitudinally of the duct 28 since sound does not project itself in single dire-ctions to form rays. It emanates equally in I all directions unless it encounters reflecting surfaces. Foraminous shell 28 allows this sound to pass freely therethrough and it is thereafter absorbed by the material in space 29.

Tests conducted with mufflers of my inven- I tion gave surprising results. Keeping constant the muflier length and the thickness of the layer of gas-pressure absorbing and sound absorbing material'adjacent the interior of the outer shell it was found that varying the diameter of the muffler within reasonable limits produced no appreciable change in ultimate-quantity of sound, measured by means of an acoustimeter, which emerged from the muffler when the latter was connected in the ordinary manner in the exhaust system of an internal combustion engine. However, when the unmuflied exhaust noises were acoustically separated from the gases diameter of the muflier.

andconducted to the muflier through an inbetter than that of'the larger device which 1 side diameter of the mufller'within limits, orv by increasing theflength of the absorber w th out changing its thickness, or by decreasing allows the gas to expand more and thus acts more efficiently upon it but acts lessefficiently upon the noises created therein. The muffler absorption for any one absorber may be 1n- -creased by increasing the thickness of the absorber Within limits, or by increasing the outthe as duct diameter within limits, W hile I have illustrated and described but one form of my invention, it is understood that I-may employ a, great many forms since I believe 'that I have invented the broad principle of'providing' a muffler with gas- 1 pressure absorbing and sound absorbing material adjacent the path of the exhaust gases and do not wish to be confined to structural details. For instance, the inner shells-may be of wire screen, or other suitable foraminous material, or may be entirely omitted as hereinbefore explained, and the generalshape of the device need not be cylindrical.

. Its cross section may be circular, rectangular, elliptical or 'of-any other form and the manner of assembling the parts may vary considerably from that described hereinbefore. The rectangular ,section is morev effective than the circular section because of the greater area of absorber exposed to the gases in proportion to the cross-sectional area of the duct. My -muffler consists essentially of a duct lined with'gas-pressur'e absorbing and sound absorbing construction which maybe .aduct lined with absorbing material without a. foraminous lining. It-may be a fibrous sound-absorbing lining combined wlth a foram'inous or perforated section of engine.

exhaust pipe orany duct conducting noiseproducing' gases. For non-pulsating noisy gases such as "are encountered in intakes of .air compressors, the cutting down of the gas pressure peaks is not a factor.

In any form of device I may choose to construct the crossseqtional area of the "outlet sh ould be large enough not ,to build up any appreciable back. pressure. -Many present mufflers assist the silencing action by con-.

st-ricting greatly the cross-sectional area of the gas outlet but this cannot be done Without building up back pressure.

' Although. published data avail-able relative to mechanics of noise production by nternal combustion engine exhausts are,

meagre, I believe the hypotheses and explanations hereinbefore advanced to explain the manner in Which my device accomplishes the muffler of exhaust or explosion noises are true. Whateverthe theories and explanations may be, the fact remains that, although a substantially straight and unobstructed path is'oifered the exhaust gases and noises they do not traverse that path unchanged but undergo a change therein which causes them to emerge almost, if not quite,'noiseless.

Mufflers which are consideredsatisfactory and usedby automobile manufacturers, show by acoustimeter measurement, a total muffler efficiency of at least percent When the internal combustion engines, for which the mufllers were designed, are run at full load. A muffler for an automobile engine which has a muffling efficiency at full load of 75 percentis exceptional. Although the muffling efficiency of satisfactory mufflers for automobiles is only 55 percent at full load, it mustbe'recognized, that in cities where this efficiency should be high, automobile engines practically are neverrun at full load..

Under light loads, characteristic of city driving, the efficiency may be well over 95 percent. Even on country roadswhere high speeds may beattained, automobile engines are seldom subjected to full load. 'On the other hand, motor boat engines and'especially out-boardmotors, are very often run at full load, so that unless the muffler-efficiency is high, that is, above 95 percent, the-exhaust noise is excessive. This noise is heard for a long distance over the water and therefore The OIdlIlZLIY outboard mois objectionable. tor muffler, although seemingly inefficient, has an efficiency of from 75 to percent at the sacrifice of some power due to back pressure Stationary internal combustion 'en-' gines and others which are run so that they are well loaded, usually must be equipped with high efficiency mufiiers.

The efficiency of the muffler is determined by measuring the noise generated by the gases escaping into the atmosphere with and withratio of the decrease in measured sound to that measured Without mufiling. The muffler etficieneyshould be determined when the muffler ish sed with the engine for which it is designed. This is essential since the size, shape, amount of absorbing material, and

other variables of the muffler are determined for each of the'T-various sizes and types of engines.

Using my invention, as hereinbefore described, 1t 1s possible to construct mufflers of over percent by varying the qu'antity'and type of'abso'rbing material and by varying. the construction as hereinbefore-described.

Such mufflers may be constructed so as to cause practically no mcrease 1n the back presout mufiiing. The percent efficiency is the varying efficiencies, from 5 5 percent to well" sure on the engine'at full load aside from that due to the surface friction of the' 'duct walls. Furthermore, such'mufllers occupy a small space, are light in weight, and may. be,

the flow ofs'aid gas therethrough, said absoirbingtwall structure comprising porous-packed material in layers of appreciable thickness whereby the noises are efiectuallysilenced.

2.1A muffler for noise-producing gas comprising a truncated-cone-shapedduct with a gas-pressure absorbing and sound. absorbing "Wall structure, said gas being adapted to enter.

at the large end of said du ct,,said. duct. ofiering'practically no resistance to the flow of said gas therethrough, said sound absorbing wall structure presenting a surface to said gas and being of such .e'fiiciency' that its aver age-sound absorption value is at least percent. i -f 3. An exhaust muifler'for internal combustion engines comprising an inner, open-ended truneated-cone-shaped, --peri:'orated' sheet metal shell, the larger end of said shell being toward the intake, and an outer coaxial nietallie/cylinder forming an annular space -be tween said cylinders, the annular space be tween said shell and said cylinder beingfilled with porous-packed,-metallic fibers.

4. A mufiler for the exhaust gas of an in-- .ternal combustion engine comprisingatrun cated-cone-sliaped duct with a gas-pressure absorbing and sound absorbing wall structurecomprising p0rouspacked 'nondiammable gas being adapted to enter the large end of said duct.

' 7.- A mufiler for the exhaust-gas of an internal combustion engine comprising a truncated-cone-shaped, perforated metal" duct forming a substantially straight passage for the gas froni the inlet of said mufiler to the outlet thereof with agas pressure absorbing and sound absorbing exterior backing comprising p'oro'us-packed steel wool," the fibers thereof occupying from 1 to 20 percent of the space occupied by said packed steel wool, said gas being adapted to enter at the large end.

of saidduct.

8. A muflier. for the exhaust gas of an internal combustion engine comprising an inner, open-ended, foraminous duct, and an outer coaxial imperforate metal shell spaced from said duct at increasingdistances from the inlet to the outlet of said duct, said space between said foraminousduct and said outer shell containing gas-pressure absorbing and sound absorbing material. i

In testimony whereof I affix my signature.

" FREDERICK H. SCHNELL.-

Y fibers; said gas being adapted to' enter at the large end of said duct, said duct-ofiering practically no resistance to the flow of sand gas therethrough, said absorbing wall pre senting a surface to saidgas, said mufller haV- ,ing a mufll'ing efficiency of at least per'cent when used with a fully loaded engine. 50 t f 5. A mufller. for-nolse-producinggas come 1 prising a truncated-cone-shaped fora'minousduct with a gas-pressure absorbing and sound 6. A m'ufller for the exh absorbing-exterior backing for said duct com- 1- prising porous-packed, non-flammable fibers, 55

said fibers weighingfrom' l to 100 pounds per cubic. foot, said gas being adapted to enter at; the largeend of said duct.

aust gas of-an in--, ternal combustion enginecomprising a truncated-cone-shaped, foraminous duct with a gas-pressure absorbing and sound absorbing exterior backing of suifici'ent'thi'clmess and porosity to cause gas pressure peaks? to be suppressed primarily by absorption in 'the interstices of said absorbing material, the 

